1. Field of the Invention
The present invention relates to a device for releasing loads from a hydraulic pressure supply pump provided in a power steering system for vehicles when the output member of the power steering system is near the end of its stroke. Relief of the loads is effected by short-circuiting between a hydraulic pressure source and an oil reservoir provided in the power steering system.
2. Description of Relevant Art
Commonly-employed types of power steering systems for vehicles are constructed such that a directional control valve is actuated by a steering force applied to the steering wheel of the vehicle to change the connection of hydraulic oil passages leading to a power cylinder to which the steering or front wheels of the vehicle are coupled via link means. In an oil hydraulic circuit including such hydraulic oil passages, hydraulic oil serving as an auxiliary power medium is circulated by means of a pump which is driven, for example, by the engine of the vehicle in which the power steering system is mounted.
The power steering system of the aforesaid type generally employs a known "open center system", in which hydraulic oil is permitted to permanently circulate in the oil hydraulic circuit when the directional control valve is in its neutral position. Therefore, an engine employing such "open center" type of power steering system undergoes minimum horsepower loss during interruption of the supply of auxiliary power to the front wheels when the directional control valve is in its neutral position. In such power steering system using the open center system, when a further steering force is applied to the steering wheel, even after the steering wheel is rotated to its extreme position with a contact portion of the link means urged against its stopper, the directional control valve is brought into a position wherein one of two hydraulic oil passages leading to the power cylinder is blocked, whereas hydraulic oil is fed to the power cylinder only through the other passage. Consequently, the pressure in the hydraulic oil circuit becomes excessively high, such that when a relief valve mounted in the hydraulic pressure supply pump is opened, the increased pressure drops suddenly and directly to the atmospheric pressure which is prevalent downstream of the relief valve, through the latter valve. Due to this sudden large drop in pressure, a large amount of heat is produced and the relief valve is subjected thereto, often causing seizure of the pump.
Further, the relief valve normally has its valve opening pressure set at a valve higher than a maximum pressure required for power steering operation. Therefore, in the known "locked position", in which the contact portion of the link means is in urging contact with the stopper, the pump is subjected to a large load corresponding to the high valve opening pressure. Such condition may result in stopping of the engine when the engine is operating in an idling state with low torque, or slippage of the V-belt for driving the pump.
To overcome the above-described drawbacks, some conventional load relief devices, or unloaders, have heretofore been employed which are designed to permit hydraulic oil as an auxiliary power medium to by-pass the hydraulic pressure supply pump for relief of loads applied thereto when the steering wheel is rotated to its extreme position. However, in such conventional unloading devices the power cylinder for providing auxiliary power is provided with by-pass valves at opposite ends thereof which are adapted to open upon sensing the contact of the contact portion of the link means with the stopper, which inevitably results in a power steering system which is large in size. Further, it is necessary to adjust the timing of opening of the by-pass valves with respect to the striking of the link means against the stoppers. Still further, during machining of the by-pass valve portions, very close tolerances must be met for prevention of oil leakage through the valve portions. Particularly, if the steering force transmission mechanism of the power steering system is a rack-and-pinion type, the power cylinder must be designed so as to be longer in stroke than a worm-and-sector type, and correspondingly smaller in piston area. The shape of the power cylinder is therefore unsuitable for mounting of by-pass valves thereon.
Known load relief devices include a type in which a four-port connection valve (four-way valve) is used as the directional control valve of the power steering system, and by-pass valves are arranged so as to be maintained in an inoperative condition within the normal changeover action stroke of the four-port connection valve. In this type of unloader device, if a further steering force is applied when the unloader is in a locked state, the valve body of the four-port connection valve is displaced over its normal stroke to open the associated by-pass valve. However, such type of unloader device, in addition to its complicated and large construction, has the further drawback that even in the event that the steering wheel is rotated so suddenly as to move the piston of the power cylinder at a rate higher than the actual hydraulic oil supply rate of the pump, the valve body of the four-port connection valve is easily displaced over the normal stroke to open the associated by-pass valve. Further, to obtain the displaceability over the normal stroke, the substantial stroke of the four-port connection valve is inevitably too large. This leads to an increased range of lost motion of the steering wheel and the steering road wheels of the vehicle with respect to each other during manual steering operation in the event of failure of the auxiliary power control mechanism of the steering system. In other words, the steering wheel has an excessive amount of play, which makes the steering operation unstable.
The present invention has been attained with a view toward effectively solving the foregoing problems, disadvantages and shortcomings attendant conventional arrangements.